Catalyses the two-electron reduction of the C2 and C31 diene system of 15,16-dihydrobiliverdin. Specific for 15,16-dihydrobiliverdin. It has been proposed that this enzyme and EC 1.3.7.2, 15,16-dihydrobiliverdin:ferredoxin oxidoreductase, function as a dual enzyme complex in the conversion of biliverdin IXα to phycoerythrobilin.